I say tell him now.

News of a baby is pretty shocking, even when it's what you're hoping for. It's pretty common for men, even when a couple is trying to get pregnant, to freak out/get quiet/act wierd when they hear the news.

I'd tell him now, in case he does react weirdly. By Valentines Day, he'll be over the shock, and you two can maybe do something special to celebrate the pregnancy. On the chance he goes grumpy on you, you don't want that to happen on V-Day.

(By the way - Congratulations!!!)